1. Field of the Invention
The invention relates to the field of semiconductor metrology, and in particular to a system and method for characterizing small trenches formed in silicon.
2. Related Art
Traditionally, semiconductor devices such as metal-oxide-semiconductor (MOS) transistors have been formed as essentially planar devices (sometimes referred to as “bulk” devices). Specifically, the gate dielectric and gate elements in such devices have been flat structures formed on the surface of a silicon wafer. However, in modern semiconductor devices, the need to achieve higher device performance in reduced die areas has led to the development of device designs that make greater use of the vertical dimension. For example, a “tri-gate” transistor can be formed by etching parallel trenches into a silicon wafer, thereby creating a “fin” of silicon between the trenches. A gate dielectric layer and a gate layer can then be formed that wrap over the surface of the fin, thereby creating a gate and channel structure for the transistor. Because the gate can apply an electric field to three surfaces of the channel region, control over current flow through the fin can be enhanced over similarly-sized bulk transistors.
Unfortunately, this trend towards vertical structures within increasingly miniaturized devices can significantly complicate measurements performed on those devices. For example, forming an array of tri-gate transistors requires that a series of trenches be formed in the silicon substrate. To provide the greatest device performance in the smallest die area, the trenches are relatively deep and narrow, which makes interior inspection of individual trenches extremely difficult, particularly because the trench dimensions are typically smaller than the wavelengths of light used in conventional optical metrology tools.
One approach to overcome this measurement difficulty is to use an infra-red (IR) spectroscopic ellipsometry technique to measure an array of trenches as a whole. For example, FIG. 1 shows an IR spectroscopic ellipsometry system 100 that includes a stage 110 for supporting a test sample 190, an IR beam generator 120, focusing optics 130, output optics 140, and a Fourier transform spectrometer (FTS) 150. Test sample 190 includes an array of trenches 191, with each trench having a width W and a depth D. The remaining material between each trench 191 has a width T. Typical trench dimensions for current dynamic random access memory (DRAM) designs could be depths D between 2 μm and 15 μm, and widths W of less than 0.5 μm.
To perform a metrology operation on test sample 190, IR beam generator 120 generates an IR probe beam 121 that focusing optics 130 directs onto test sample 190. IR probe beam 121 includes light from the short to long IR regions having wavelengths across the range of 2 to 20 μm. IR probe beam 121 is reflected by test sample 190 as an output beam 122 that is directed by output optics 140 onto FTS 150. A polarizer 131 in focusing optics 130 and an analyzer 141 in output optics 140 allow polarization changes caused by test sample 190 to be measured by FTS 150.
To analyze the data gathered by FTS 150, the array of trenches 191 is modeled as a “fictitious” film of a material having characteristics falling somewhere between those of air and those of pure silicon. This fictitious film modeling can be performed due to the fact that silicon is essentially transparent to IR light, so that any reflection that occurs (i.e., output beam 122) is a function of the geometry of trenches 191. Furthermore, because the wavelengths of IR probe beam 121 will generally be much larger than the width of trenches 191, output beam 122 will include contributions from many trenches 191. Therefore, the resulting measurements will be representative of the array of trenches as a whole.
Once the characteristics of the fictitious film are determined, those characteristics can be used to generate measurement values for trenches 191. For example, the thickness of the fictitious film can be provided as the measured value for the depth D of trenches 191. Likewise, the index of refraction for the fictitious film can be used to determine a volumetric ratio for the trench (air) and silicon regions, thereby allowing an average width W for trenches 191 to be determined. In this manner, IR spectroscopic ellipsometry system 100 can enable characterization of the array of trenches 191 on test sample 190.
Unfortunately, IR spectroscopic ellipsometry system 100 is faced with several issues that limit its applicability and effectiveness as a metrology tool. First, due to the low reflectivity of silicon to IR light, it is desirable for IR probe beam 121 to have a high intensity to maximize the signal provided by output beam 122. However, the relatively long wavelengths of light in IR probe beam 121 (i.e., low to high IR wavelengths) can make generation of a high intensity difficult. Furthermore, those long wavelengths can also prevent measurements from being taken in small regions of a test sample (e.g., in the scribe line of a wafer), thereby limiting the flexibility of IR spectroscopic ellipsometry system 100.
Finally, the main issue affecting the feasibility of IR spectroscopic ellipsometry system 100 as a production tool is the high cost and complexity of the components required for IR signal processing. For example, due to chromatic aberration effects that increase with increased wavelength, high-precision mirrors must generally be used in focusing optics 130 and output optics 140, rather than more inexpensive lenses. In addition, because typical CCD (charge coupled device) and diode arrays are unable to sense wavelengths above roughly 900 nm, IR spectroscopic ellipsometry system 100 requires the use of FTS (Fourier transform sensor) 150. FTS 150 incorporates beam splitting optics 151 and an interferometer 152 that allows a Fourier transform to be performed on output beam 122 to generate a spectrum of intensities for the range of wavelengths in output beam 122. FTS 150 is a very complex and expensive sensor that is also very delicate and extremely sensitive to vibrations, and is therefore not particularly well-suited for a production environment.
Accordingly, it is desirable to provide a robust, reliable, and low-cost system and method for performing metrology on trenches in a silicon substrate.